Strip profiles have many common shapes identified as flat or rectangular, heavy center or convex light center or concave. Often it is desired to produce finished metal strip having a convex profile. Further it is not just the convex, profile that is important, but it is the shape of the convex profile that is critical. To this end, it is often desirable to produce a convex profile that is polynomial. In other words, the convex profile, specifically the curvature of the top and bottom edges, can be described mathematically by a polynomial function.
Obtaining a convex profile that is polynomial is typically performed by at least one of the two known methods, roll bending or roll shifting. Roll bending refers to placing load on the journaled ends of the work rolls of a mill stand, typically only the top work roll, in order to bend the work rolls, and thus to modify the metal strip profile.
The basic functions of positive roll bending are to increase the reduction at the center of the strip and to reduce the reduction at the edges of the strip. Conversely, negative work roll bending gives increased reduction at the edges of the strip and can lead to a decrease in the reduction at the center of the strip.
The other way to correct the profile of a metal strip is by roll shifting which refers to axially shifting at least one non-cylindrical roll in the mill stand. Axially shifting at least one non-cylindrical roll, changes the shape of the space between the work rolls. This space between the work rolls defines the roll gap. Changing the roll gap by roll shifting can also cause the "correction" of a strip profile to create a polynomial profile. Correcting a strip profile involves altering the curvature of the surfaces of the metal strip without changing the gauge of the strip. The change in the strip profile is dependent on the shape of the roll, work roll, intermediate roll or backup roll, that is shifted. Not all roll shapes or combinations thereof can create a roll gap that will correct a strip profile to produce a polynomial profile. Correction of strip profile by roll shifting is dependent on the shape of the non-cylindrical roll or rolls that are shifted as well as the shape of the strip profile to be corrected.
Roll bending and roll shifting create various strip profiles. Various strip profiles created on a rolling mill by roll bending and roll shifting are referred to as a family of strip profiles. A family of strip profiles comprise a strip profile envelope. The greater the strip profile envelope the greater the capability of the mill to produce desired profiles.
One example of prior art roll shifting is the so-called continuously variable crown, or CVC, rolling in which the work rolls and backup rolls have an S- or bottle-shaped profile which provides for adjustment of the roll gap profile by bi-directional shifting of the rolls. Disadvantages of the CVC system are that it requires special, asymmetrical roll grinding, and produces an asymmetrical backup roll wear pattern. Moveover, it does not provide sufficient improvement to avoid the need for use of several sets of rolls for rolling a range of sheet or strip of various sizes which can be rolled in a given mill.
When the material is rolled between the curved initial crown portions of the upper and lower work rolls, a variation of the roll gap is small even if the upper and lower work rolls are axially shifted, and by compensating for this variation by roll bending, the work rolls can be cyclically shifted axially within a predetermined range. By doing so, the wear of the work rolls due to the rolling is dispersed, the initial crown of the work rolls can be maintained for a long period of time. As a result, it is possible to perform the rolling operation of the wide material after- the rolling operation of the narrow material is performed, and the limitation on the order of the rolling operation with respect to the width of the material to be rolled can be eliminated.